A method in which the top face of a piston made from an aluminum alloy is subjected to an anodizing treatment to form an anodic oxide coating, and the surface of the anodic oxide coating that is formed as a result is subjected to a sealing treatment is already known. For example, in Japanese Patent Laid-Open No. 2012-72745, a method for manufacturing a piston is disclosed that includes a step of forming a porous layer by subjecting the top face of a piston made of an aluminum alloy to an anodizing treatment, and a step of forming a coating layer that covers the surface of the porous layer by plasma spraying of a Y2O3-stabilized ZrO2 powder onto the porous layer. Similarly to a common anodic oxide coating, the porous layer has a large number of pores that are formed during the course of the anodizing treatment, and has a lower thermal conductivity and also a lower thermal capacity per unit volume than a conventional ceramic-based heat-shielding coating. Further, the coating layer is formed so as to block the openings of the pores of the porous layers, and the Y2O3-stabilized ZrO2 has a lower thermal conductivity than the aluminum alloy. Therefore, according to the heat-shielding coating constituted by the coating layer and the porous layer, a low thermal conductivity and a low thermal capacity per unit volume can be realized.
Further, in Japanese Patent Laid-Open No. 2010-249008, a technique is disclosed that forms a silicon oxide coating on the top face of a piston by coating an organic silicon solution onto the outermost surface of an anodic oxide coating formed on the top face of the piston, and performing a heat treatment. Similarly to the above described coating layer, openings of pores in the anodic oxide coating can be blocked by the silicon oxide coating. A low thermal conductivity as well as a low thermal capacity per unit volume can also be realized by this kind of heat-shielding coating constituted by a silicon oxide coating and an anodic oxide coating.